Electrically programmable read-only memories (EPROMs) and electrically programmable electrically erasable read-only memories (EEPROMs) are non-volatile semiconductor memory devices based on metal oxide semiconductor field effect transistors (MOSFETs). EPROM and EEPROM cells store a bit of information as a quantity of electrons on a floating gate structure which is insulatively disposed between the channel and the control gate of a field effect transistor. A charged floating gate raises the threshold voltage of the field effect transistor channel above the voltage normally applied to the control gate during the read operation such that the transistor remains shut off when read voltages are applied to the gate, source and the drain, thereby returning a logical "0". An uncharged floating gate does not alter the threshold voltage of the channel of the field effect transistor, and therefore a normal gate reading voltage will exceed the threshold voltage, turning on the transistor when read voltages are applied to the gate, source and the drain. In this condition, a logical "1" is returned.
When EPROM or EEPROM cells use a conventional floating gate avalanche injection metal oxide semiconductor (FAMOS) structure, the floating gate is charged by avalanche injection, commonly referred to as "hot electron injection". Prior structures have allowed for source side injection where a majority of the electrons injected to the floating gate come from the source side of the cell. In these structures, a much more significant gate current can be realized since the electric field across the gate oxide near the source, as created by the voltage difference between the grounded source and the control gate is at a maximum. One such source side injection cell is disclosed in an "Asymmetrical Non-volatile Memory Cell, Arrays and Methods for Fabricating Same," filed by Liu et al. on Aug. 29, 1990, application Ser. No. 07/575,105 assigned to the assignee of the present application, now abandoned, the disclosure of which is hereby incorporated by reference. The structure disclosed in the previously cited application shows significantly enhanced gate current due to the high efficiency of source side injection. The structure allows for fast programming speed at 5 volts on the drain due to the enhanced injection efficiency. Furthermore, the structure has the potential of being programmed at 3.3 volts on the drain, thus allowing the operation of this structure with scaled power supplies. However, the structure disclosed in the previously cited application requires a relatively high gate voltage to enable fast programming.
Accordingly, a need has arisen for non-volatile memory cell which allows for fast programming through the operation of source side hot electron injection at lower gate voltages.